Escherichia coli have been recognized as an important human pathogen and major cause of diarrhea and hemorrhagic colitis. E. coli can be characterized into several different categories based on pathogenic features, including enteropathogenic (EPEC), enteroaggregative (EAEC), enteroinvasive (EIEC), enterotoxigenic (ETEC), and Shiga toxin-producing (STEC). Enterohemorrhagic E. coli (EHEC) are a subgroup within the STEC and these strains cause hemorrhagic colitis and severe disease in humans.
There are essentially two main types of Stxs: Stx/Stx1 and Stx2. Stx is produced from Shigella dysenteriae type 1, while Stx1 and Stx2 are produced from Escherichia coli. Stx and Stx1 are virtually identical, with only one amino acid difference in the A subunit. The mature A and B subunits of Stx1 and Stx2 have 68 and 73% similarity, respectively.
The majority of previous studies have focused on E. coli O157:H7 because this strain was most commonly linked to illness in the United States in the 1980's and 90's. However, outside of the United States non-O157 strains are often more prevalent and recently non-O157 strains have been linked to outbreaks and illness in the United States.
Cattle have been identified as potential reservoirs for E. coli and consequently raw milk and under-cooked ground beef have been implicated as sources of human infection. In 1994, the Food Safety and Inspection Service (FSIS) under the authority of the Federal Meat Inspection Act declared E. coli O-157:H7 to be an adulterant in ground beef and over the subsequent years new and revised protocols for O157:H7 testing in ground beef have been implemented. In 2000, non-O157 STEC became a reportable disease and there was a voluntary request for public health departments to report cases to the National Notifiable Diseases Surveillance System. A study conducted on non-O157 STEC infection in the United States from 1983 to 2002 found that 940 non-O157 STEC isolates had been submitted and confirmed by the Center for Disease Control and Prevention (CDC). The majority of the strains (71%) belonged to one of six major serogroups, including O111, O26, O103, O145, O45, and O121. Current research is focusing on these six strains to better understand the epidemiology in humans and cattle and develop accurate testing procedures. Understanding the epidemiology of these six strains is vital to developing methods to detect food products that been infected with Shiga toxin-producing E. coli strains.
It is difficult to report on the prevalence of non-O157 E. coli infections in the United States inasmuch as reporting is not mandatory. The non-mandatory reporting compounds the information gap for the various E. coli serogroups. The association between strains found in cattle and those causing human illness is further complicated because not all non-O157 E. coli found in cattle cause disease. Many strains of non-O157 E. coli in cattle lack Shiga toxin genes and therefore are not virulent to humans. It will be important to both the cattle industry as well as consumers to develop tests that not only identify non-O157 E. coli but also differentiate between virulent and non-virulent strains.
Given the inability to detect non-O157 E. coli Shiga toxin-producing serotypes, there is a need to develop constructs and methods to detect such serotypes to ensure food safety, particularly in vegetable produce and animal meat products. In order to prevent unnecessary loss of product and revenue, such test and assays that detect these serotypes need to be both fast and accurate, preferably in conjunction with RT-PCR techniques.